I. Field of the Invention
The present invention relates to the fabrication of powder compact bodies and particularly such compact bodies suitable as contacts in vacuum current interrupters, other plasma devices and the like, and, more particularly, to a method and apparatus for the fabrication of such contacts from a powder material wherein various desirable properties of the contact material are optimized.
II. Description of the Prior Art
Contacts for vacuum current interrupters and the like are presently fabricated using the well known techniques of vacuum melting and vacuum infiltration. Such contact-forming processes are specifically designed to optimize various operating charcteristics of the resulting contacts. These characteristics include a low gas content and high thermal and electrical conductivities. The contact must be able to withstand the high current arcs encountered on interruption and exhibit a low chopping current level. Antiwelding characteristics are also desirable for preventing the contacts from welding together upon completing a circuit. Generally, a single material or element does not possess all of these desirable properties and a compromise characteristic is presently obtained by forming an alloy or mixture of a high conductivity metal such as copper and/or silver, and a minor component of a relatively high vapor-pressure conductive material, i.e., a brittle metal such as bismuth, antimony, and/or arsenic. Vacuum melting is employed to produce a true alloying, i.e., formation of a solid solution. There are certain disadvantages to vacuum melting. Some of these disadvantages are as follows:
1. Little, if any, control of true alloying is possible. Other physical properties, for example, melting point and wettability of the several metal constituents or components, may make complete melting and/or coalescing extremely difficult.
2. The difference in component densities in multiple component contact bodies, and an inadequate mixing or stirring during formation may create a non-uniform component distribution with segregation into layers.
3. The evaporative losses of different components may vary, making precise quantitative control of the component composition difficult.
4. Undesirable interactions may occur among some of the contact components and between the melt and the melting apparatus. An example of such component interaction occurs where the melt includes copper and small amounts of magnesium fluoride which may react to form copper fluoride. An instance of the second interaction may arise where a conventional graphite crucible is employed to contain a melt of copper and zirconium which when melted, reacts with the graphite to form a copper/zirconium/zirconium-carbide body upon solidification.
5. The grain structure of the contact resulting from the vacuum melting process may be of a type which produces defects such as cracks, laminations and asperities.
6. In vacuum melting, the solidification generally creates a "shrink" hole in the upper surface of the body which must be removed as wasted material. The solidified contact body further requires substantial machining operations to form a finished contact.
7. While a properly solidified vacuum melt contact tends to have a highly desired low porosity, the process does not provide control of this property.
8. The solidified contact formed by this method is not a finished component and may for example require substantial machining operations with the attendant expense and possible damage to the contact as a result of the presence of a brittle component.
Suitable contacts for vacuum current interrupters and/or plasma devices have also been formed with conventional powder/metallurgy techniques wherein a powder is first subjected to high compaction pressures and only thereafter heated to sintering temperatures. Although many problems associated with the vacuum melting method may be avoided, other problems arise, which are typically as follows:
1. Generally, the resulting compact bodies have appreciable residual porosity unless ultra pure powders are used, and a series of extreme procedures, such as very high initial compaction pressures in special multi-action presses with floating dies and the like and very high sintering temperatures are employed.
2. The resulting compact bodies tend to have somewhat higher gas content and may actually explode during initial sintering due to entrapped gases. The compact bodies also are more likely to have body defects, such as cracks, and laminations.
3. Cold compaction tends to work harden the compact body being formed such that densification is increasingly retarded and finally stopped.
4. Friction between the outer compact body surfaces and the die wall and die plunger results in non-uniform density distributions making formation of compact bodies with large length-to-diameter ratios with uniform density virtually impossible.